Ink-jet recording processes are recently attracting attention. Ink-jet recording is a method of printing in which small droplets of an ink composition are ejected and adhered to a recording medium, e.g., paper, to conduct recording. This method is characterized in that high-resolution images of high quality can be printed at a high speed with a relatively inexpensive apparatus. Ink-jet recording apparatus, on which recording is conducted by this method, have found extensive commercial acceptance because of image quality, low cost, relatively quiet operation, and the ability to form graphic images. Of these, thermal (Bubble Jet (trademark)) and piezoelectric drop-on-demand printers have especially succeeded in the market and have been in wide use as printers for personal computers in offices and homes.
Recently, a technique has come to be used in which two or more color ink compositions are used to form color images by ink-jet printing. In general, color images are formed with inks of three colors composed of a yellow ink composition, a magenta ink composition, and a cyan ink composition or, in some cases, with inks of four colors which include a black ink composition besides these three ink compositions. There also are cases where inks of six colors which include a light-cyan ink composition and a light-magenta ink composition besides ink compositions of these four colors or inks of seven colors which further include a dark-yellow ink composition besides ink compositions of these six colors are used to form color images. Such ink compositions for use in forming color images each are required not only to give a satisfactory color by itself but also to give a satisfactory intermediate color when used in combination with other ink composition(s). The ink compositions are further required to give printed matters which do not suffer a color change or fading during storage.
In recent years, printed matter obtained by “photographic-image-quality” printing by color ink-jet printers have come to compare favorably with “silver salt photographs” as a result of continuous improvements in heads, ink compositions, recording methods, and media. Namely, ink-jet printing has become “comparable to photographs” in image quality. On the other hand, improvements in the storability of images obtained are being attempted by improving ink compositions and media. In particular, light resistance has been improved to such a level as to pose no practical problem (see patent documents 1 and 2 identified below). However, the light resistance has not reached the same level as that of silver salt photographs. In a standard method for evaluating light resistance, the degrees of fading of pure-color patterns respectively of Y, M, and C (optical density, around 1.0) are used as indexes for judgment. Among the ink compositions mounted on printers commercially available presently, the magenta ink compositions have the lowest light resistance when judged by the above-described evaluation method. This light resistance of magenta ink compositions, in many cases, governs the light resistance life of ink sets. Consequently, improving the light resistance of magenta ink compositions leads to an improvement in the light resistance of photographic images and to prolongation of the light resistance life of ink sets.
On the other hand, it has become possible to obtain images free from graininess by using an ink set including two ink compositions differing in color density. In such ink sets including two ink compositions differing in color density, which are intended mainly for photographic image printing, ink compositions having a low color density are generally used frequently for photographic image formation for the purpose of diminishing/eliminating graininess. With respect to the evaluation of light resistance described above, patterns having an optical density around 1.0 are ones formed from ink compositions having a low color density. Consequently, improving the light resistance of ink compositions having a low color density leads to an improvement in the light resistance of photographic images and prolongation of the light resistance life of ink sets. In addition, since ink composition having a high color density are also used in forming richly colored images or graphic-art-like patterns, improvements of these ink compositions are also useful.
Patent document 1: JP-A-2000-290559
Patent document 2: JP-A-2001-288392